Deep Impact mission reveals comet's icy cargo

Water ice is present on the surface of Comet Tempel 1, suggest observations from NASA's Deep Impact mission. This is the first direct detection of exposed water ice on a comet.

But the mission's science team says the water ice is present in surprisingly small amounts, covering less than 1% of Comet Tempel 1's surface. The finding suggests the comet's surrounding cloud of gas and dust may largely be fed by underlying ices, rather than by gas streaming off its surface.

Old assumptions about comets are faltering as results emerge from data collected by the Deep Impact spacecraft in July 2005, when the probe's impactor detached from the mother ship and crashed into the comet at 37,000 kilometres (23,000 miles) per hour.

Combining high resolution images and infrared spectra collected during the probe's approach, a team of nearly two dozen scientists pinpointed three patches of water ice on the surface of the comet's "upper" half.

The team also found the comet was much weaker structurally than previously believed; the soufflé-like comet is more empty space than rock and ice.

The consensus model of a comet leading up to the Deep Impact experiment is no longer valid, says Don Yeomans at NASA's Jet Propulsion Laboratory in Pasadena, California, US, a member of the mission science team. "It's certainly not a dirty iceball or an icy dirtball," he told New Scientist. "It's a very, very weak, dusty structure with interior ices."

What lies beneath

Deep Impact struck an ice-free surface area on Tempel 1, says Jessica Sunshine of Science Applications International Corporation in Chantilly, Virginia, US, who led the new study.

But in analysing the ejecta from the comet after impact, she says, "one of the first things we saw was water ice". This indicates that while water ice is not at the comet's surface, it lies just beneath, within its upper metres.

The team says the surface deposits may be responsible for some of the comet's natural emissions. "But what is perhaps even more interesting is that most of the [emission] jets are not due to surface ice, they're due to subsurface ices that somehow permeate that surface," says Yeomans.

Smaller and smaller

That observation may in turn answer another long-standing question about comets: why do some comets seem to be "turned off", or dormant.

If sunlight must penetrate the dust covering a comet's water ice in order to warm it and produce jets, Sunshine says the Deep Impact findings suggest the ices on such dormant comets may not have run out but merely become sealed - by layers of debris, for example.

One thing is clear, Sunshine says. "People used to think of comets as just a thing that simply sat there and got smaller and smaller [as its water ice evaporated]. But it's clearly more complex than that."

Journal Reference: Science (DOI: 10.1126/science.1123632)

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